describe and carry out investigations using whole plants, including aquatic plants, to determine the effects of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis

Investigation of Limiting Factors in Photosynthesis

Learning Objective

Describe and carry out investigations using whole plants, including aquatic plants, to determine the effects of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis.

Key Concepts

• Photosynthesis converts light energy into chemical energy, producing O₂ and carbohydrates.
• The overall equation can be written as

  \$\text{6 CO}2 + \text{6 H}2\text{O} \xrightarrow{\text{light}} \text{C}6\text{H}{12}\text{O}6 + \text{6 O}2\$

• Rate of photosynthesis is commonly measured by the volume of O₂ produced or the amount of CO₂ consumed.
• Limiting factors are environmental variables that, when altered, change the rate of photosynthesis until another factor becomes limiting.

Variables in an Investigation

General Experimental Setup

1. Select a suitable whole plant or a detached leaf (e.g., Elodea for aquatic studies, spinach leaf for terrestrial studies).
2. Place the plant in a sealed transparent container filled with water (or appropriate medium) to allow gas exchange measurement.
3. Introduce a delivery system for the independent variable (e.g., variable‑intensity lamp, CO₂ syringe, water bath).
4. Collect the gas produced in an inverted graduated cylinder or a gas syringe.
5. Record the volume of O₂ evolved at regular time intervals (e.g., every minute for 10 min).

Investigating Light Intensity

Apparatus

• Elodea sprigs or spinach leaf
• Transparent water bath with a graduated cylinder
• Variable‑intensity lamp (adjustable distance or dimmer)
• Lux meter (optional for quantitative light measurement)
• Thermometer

Method

1. Place the plant in the water bath and ensure it is fully submerged.
2. Position the lamp at a fixed distance and record the initial light intensity (e.g., 500 lux).
3. Start the timer and record O₂ volume every minute for 10 min.
4. Repeat the experiment at increased light intensities (e.g., 1000 lux, 1500 lux, 2000 lux) by moving the lamp closer or increasing the voltage.
5. Maintain water temperature constant (±1 °C) throughout all trials.

Sample Data Table

Analysis

Plot O₂ volume against time for each light intensity. The slope of the linear portion gives the rate of photosynthesis (\$\text{rate} = \Delta V{\text{O}2}/\Delta t\$). Expect an initial increase in rate with light intensity, reaching a plateau when other factors become limiting.

Investigating Carbon Dioxide Concentration

Apparatus

• Elodea sprigs
• Sealed container with gas inlet/outlet
• Syringe or gas burette delivering known volumes of CO₂
• Light source of constant intensity
• Thermometer

Method

1. Set up the plant in the container under a fixed light intensity (e.g., 1500 lux).
2. Introduce a known concentration of CO₂ (e.g., 0 %, 0.5 %, 1 %, 2 % by volume) by injecting the gas and allowing equilibration for 2 min.
3. Measure O₂ evolution as described previously for 10 min.
4. Keep temperature constant (e.g., 25 °C) throughout the experiment.

Sample Data Table

Analysis

Calculate the rate of O₂ evolution for each CO₂ level. A graph of rate versus CO₂ concentration typically shows a hyper‑bolic relationship, approaching a maximum when the enzyme Rubisco becomes saturated.

Investigating Temperature

Apparatus

• Elodea or aquatic plant in a water bath
• Thermostatically controlled water bath (e.g., 10 °C, 20 °C, 30 °C, 40 °C)
• Constant light source (e.g., 1500 lux)
• Gas collection apparatus

Method

1. Place the plant in the water bath and allow it to equilibrate at the target temperature for 5 min.
2. Start gas collection and record O₂ volume every minute for 10 min.
3. Repeat for each temperature setting, ensuring light intensity and CO₂ availability remain constant.

Sample Data Table

Analysis

Plot rate of O₂ evolution against temperature. The curve typically rises to an optimum (around 30 °C for many temperate plants) and then declines due to enzyme denaturation.

Safety Considerations

• Handle hot water baths with care to avoid burns.
• When using CO₂ cylinders, secure the regulator and avoid over‑pressurising the container.
• Dispose of plant material responsibly after the experiment.

Conclusion Checklist

1. Identify which factor was limiting under the experimental conditions.
2. Explain the observed trend using enzyme kinetics and the light‑dependent reactions.
3. Discuss how the results relate to natural environments (e.g., shade tolerance, seasonal temperature changes).

Extension Questions

• How would the results differ if a C₄ plant were used instead of a C₃ plant?
• Design an experiment to investigate the combined effect of two limiting factors simultaneously.
• Explain how the concepts of \$Km\$ and \$V{\max}\$ from enzyme kinetics apply to the CO₂‑response curve.